A service is a component that runs in the background without user interaction and with no user interface.
Services performs long running tasks without being visible like, playing music, triggering notifications etc.
Services has two forms:-

1) Started Service :-  A service is called started service when an application component (like activity) starts the service by calling "StartService()" .
Once Started a service runs indefinitely , even if the starting component gets destroyed.

LifeCycle of a Started Service:-

StartService()  ->  onCreate()  ->  onStart()  ->  onDestroy()

 2) Bound Service :- A service is called a "bound service" when an application component binds to it by calling bindService().

LifeCycle of a Bound service:-

StartService()  ->  onCreate()  ->  onBind()  ->  onUnBind()  -> onDestroy()
    

Here is the simple example that makes you more clear about the services:-

Step :-1 create a service class:-

public class MyServiceOne extends Service {
    @Override
    public IBinder onBind(Intent intent) {
        System.out.println("On Bind");
        return null;
    }
    @Override
    public void onStart(Intent intent, int startId) {
      super.onStart(intent, startId);
      String msg = intent.getStringExtra("MSG");
      System.out.println("Message is- "+ "On Service start(),I got msg "+msg);
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
            System.out.println("On Destroy");
    }

Step 2:- Create a MainActivity that call the service and code the below lines to call the service

Intent intent=new Intent(this,MyServiceOne.class);
intent.putExtra("MSG","Mesaage started from Main activity");
startService(intent);

Step 3:- You must declare a service in manifest:-

<service android:name=".MyServiceOne"/>